Carlos A. S. Gomes, Bin Hu, James D. Freihaut

Transcription

1 RESUSPENSION OF ALLERGEN-CONTAINING PARTICLES UNDER MECHANICAL AND AERODYNAMIC FORCES FROM HUMAN WALKING - INTRODUCTION TO AN EXPERIMENTAL CONTROLLED METHODOLOGY Carlos A. S. Gomes, Bin Hu, James D. Freihaut Indoor Environment Center, Architectural Engineering Department, Penn State University University Park, PA 1680, USA ABSTRACT Allergens originating from mites, insects, animal dander and fungal spores are found in building surface reservoirs such as floors, upholstery and beds. Epidemiological evidence indicates that these allergens are strongly associated with the development of bronchial hyperreactivity (BHR), or asthma. Although life threatening in rare occasions, asthma affects nearly 50% of the population in developed societies, resulting in much distress and lost time from school and work. These diseases are believed inhalation sensitized and developed over time. This fact suggests the existence of an aerobiological pathway of allergen-containing carrier particles from building reservoir to occupant respiration. In this study, a reliable and reproducible methodology was developed to explore the influence of human walking on the aerosolization of allergen-containing particles. Time resolved particle size distribution and allergen content will be measured for particles resuspended from representative samples of flooring materials and for different sets of floor disturbances in an environmentally controlled experimental chamber. Initial results, when placed in the context of previous investigations, indicate the method can be utilized to develop a database for particle resuspension rates. 1. INTRODUCTION Epidemiological studies indicate continuing significant increases in health care and hospitalization of patients for respiratory system related diseases such as asthma. According to the Center for Disease Control and Prevention [1], approximately 6% of all Americans suffer from asthma and approximately 5,000 people die each year of asthma or related complications. The economic burden of this illness in the United States is estimated at $1.7 billion dollars per year []. Symptoms of asthma may be triggered in genetically predisposed individuals and developed in non-atopic individuals by exposure to allergens. Common indoor allergens are found in cat and dog fur or saliva, cockroach and dust mite droplets and body parts. After disintegrating, these materials adhere to inert dust in carpets, upholstery, and other reservoir surfaces making allergens available for secondary aerosolization when disturbed by human activity (walking, vacuum cleaning, etc.). Specific allergens associate with different ranges of carrier particle sizes [3] and after re-entrainment can stay airborne for relatively long periods of time as respirable particles. The first studies examining the impact of human activity on particle resuspension were conducted in the 1960 s with radioactive material from the floor of nuclear facilities. The resuspension factors from these studies and from more recent ones were summarized by the US Nuclear Regulatory Commission [4] and ranges from 6x10-8 to 7x10-4 m -1. Resuspension factor (RF) is defined as the ratio between airborne particle concentration and surface particle concentration, as shown in equation airborne concentration (#/ m ) RF ( m ) = (1) surface concentration (#/ m ) A second generalized form to express resuspension is by using a resuspension rate (RR), which is defined as the fraction of a surface species removed in unit time. The definition is described in equation. 1 surface removal rate (#/( m. s)) RR (min ) = () surface concentration (#/ m ) Sehmel [5] presented a summary of particle resuspension factors caused by mechanical disturbance from indoor human activities (walking and sweeping) and outdoor activities (pedestrian walking and vehicular traffic), ranging from 1x10-10 to 3x10 - m -1. Until the

2 1990 s, occupancy-related particle resuspension in residential and offices buildings was seldom explored. Extrapolated resuspension values from the previously nuclear material studies had very limited application. More recently, due to the increase of particulate-related respiratory diseases (in particular asthma) and the emergence of CBW attacks on civilian populations, several studies on indoor particle resuspension have been performed [6,7,8,9,10,11,1,13]. Bacteria-carrying particles, baccilus subtilus spores, penicillium chrysogenum spores, grass pollen and indoor inert dust were dispersed over representative indoor flooring such as carpet, wood or vinyl and disturbed by human activity like walking, working at a desk, vacuuming, or mopping. These studies were conducted in experimental rooms, hospital operation room and residential buildings and seldom with ambient conditions control. The RF from these studies ranged from 10-5 to 10-1 m -1 and the RR from 10-8 to 10-1 min -1. Allergen concentrations on home floors as well as allergen concentration in the air for quiescent and disturbed conditions have also been measured and their ranges are represented in Table 1 [14,15,16,17]. Table 1. Reservoir and air allergen concentration The absence of controlled parameters in the experiments of previous studies limits their application in the health risk assessment. Environmental conditions, such as humidity, were rarely considered or controlled to isolate their importance in particle resuspension. Systematic parametric variation, such as floor type, dust type and load, contaminant concentration in dust load, have not been performed, leading to difficulties in interpreting available data. Although it is generally accepted that floor disturbance mechanisms are mechanical, aerodynamic and electrostatic in nature (see figure 1), there is no consensus, explanatory theory for particle resuspension that can predict the effects of floor surface disturbances on particle re-entrainment. Figure 1. Reservoir disturbance due to people walking This report describes the development of an experimental and analytical methodology to examine particle surface-to-air aerosolization when reservoirs are subjected to human-related disturbances. The purpose is to establish a data bank detailing particle resuspension as a function of floor vibrations and transient near surface air flows, characteristic of human walking. This methodology was tested by conducting a set of resuspension experiments using carpet and linoleum flooring loaded with reference quartz and German roach dusts.. RESUSPENSION CHAMBER METHOD The methodology purpose on this research is presented in figure. Figure. Particle resuspension experimental measurement flow diagram Experiments are conducted in a constructed experimental chamber (400x00x00 mm) with temperature and relative humidity control. Dust samples are prepared containing various types of allergen (Bla g

3 1, Bla g, Der p 1, Der p, Can f 1 and Fel d 1) separated into precise particle size and allergen concentration ranges. The prepared dust is uniformly deposited on typical building floor samples (size: 90x90 mm) using designed particle disperser equipment. The selected surface samples are subjected to computer controlled levels of aerodynamic and mechanical disturbances, simulating the disturbance conditions associated with human walking. Resuspended particles are carried by a particle-free cross air flow and sampled by an optical particle counter (counts number of particles/v air in 8 different bins ranging from 0.3 to > µm) and a cascade impactor (8 separate impactor collectors for particle diameters ranging from 0.4 to >9 µm). This air flow sample provides time resolved particle size distribution and size resolved samples to be tested for allergen concentration using ELISA assay techniques [18]. The aerodynamic disturbance was simulated with the impingement of six small air jets over the flooring sample. To understand the walking-related airflow motion nearby the floor, experiments were developed in a close environmental chamber using CO vapor released over the floor [19]. These experiments provided the range of horizontal air velocity and the visualization of large scale air turbulence resulting from a human walking. The mechanical disturbance was simulated with a system that replicates field collected floor vibration data caused by human walking. Floor vibration acceleration generally falls between 0 and 5% of g (gravitational acceleration) with frequencies ranging from 4 to 0 Hz [0,1,,3]. However, it is not uncommon to find accelerations higher than 70% of g [4]. Electrostatic built-up voltage caused by the shoes/floor interaction can reach values higher than 10,000 Volts [5] and can potentially interfere with surface-to-air particle aerosolization, in particular organic or organic-containing material. This phenomenon is not presently incorporated on this research, but will be include in the near future. 3. RESULTS Calibrated quartz particles of known density, size distribution and composition are used to establish a basis of comparison for resuspension behavior with laboratory-produced, German roach dust particles. Two types of flooring, plastic carpet (100% olefin) and linoleum, were utilized. The floor samples were uniformly loaded with 50 mg of the two types of dust. The resuspension chamber temperature was kept in between 6 C and 8 C and the relative humidity kept constant at 45%. Three sets of floor disturbance were implemented: (1) floor vibration, () air puff and (3) combination of both. For comparison, clean flooring samples were tested to the same set of disturbance and revealed no particle resuspension. Figure 3 shows the vibration and aerodynamic floor disturbance signal used. Floor acceleration x [m/s ] approximating Floor Disturbance Signals passing Floor w alks aw ay Time (sec) & Air puffs Air Puff Figure 3. Floor vibration and air puff disturbances signals The disturbance lasted 10 minutes, but significant particle resuspension occurred only for the first two minutes. After the disturbance had begun, a practically instantaneous burst of resuspended particles was observed. As the disturbance continues, the resuspended particle count decreased exponentially for about two minutes, returning to the chamber background particle concentration values. Beyond the second minute, even with dust over the floor samples, there was little further particle resuspension. Figure 4 and 5 shows the peak and average resuspension factors (RF) and rates (RR) measured for particles with diameter > µm.

4 1.0E E-0 1.0E-03 Peak RF [m -1 ] Peak RR [min -1 ] Figure 4. Results - Peak RF and RR 1.0E-03 Avg RF [m -1 ] Avg RR [min -1 ] 1.0E-09 Figure 5. Results - Average RF and RR The average RF and RR are values integrated over every second of the measuring period for two minutes after the disturbance started. The peak RF and RR are values determined for the period of one second when the highest particle concentration was observed by the particle counter. 4. DISCUSSION AND CONCLUSIONS Peak RF and RR measured in these experiments ranged from 10-6 to 10-3 m -1 and 10-5 to 10 - min -1, while average RF and RR ranged from 10-8 to 10-4 m -1 and 10-7 to 10-3 min -1, respectively. Despite the four order of magnitude range, these values fall between field measured values found in the literature review. The main observations derived from the experiments performed were: (1) for a continuous disturbance, resuspension was only observed during the first two minutes with an initial burst of particle reentrainment followed by an exponential decrease to undetectable value; () air-puff disturbances had a much higher impact on dust resuspension than the vibration disturbances; (3) particles were more easily resuspended from linoleum flooring than from carpet flooring; (4) German roach dust was more easily resuspended by air streams than quartz dust; (5) RF and RR values derived from the present experiments show consistency with previous research values The methodology presented has been demonstrated and proven to be a valuable tool to gather reliable information on particle resuspension. The controlled environmental and disturbance conditions, the flexibility to generate different types of disturbances (including a future electrostatic disturbance), the broad range and flexibility of air sampling, the flexibility to use different flooring and different dust such as allergen containing dust and surrogate CBW dusts make it a potential useful tool for particle resuspension research and thereby contribute to the development of exposure risk models. 5. ACKNOWLEDGEMENTS The authors thank the Pennsylvania State University Institutes of the Environment and the Indoor Environment Center for financial support. 6. REFERENCES [1] Center of Disease Control and Prevention, Department of Health and Human Science, [] National Library of Medicine, United States National Institute of Health, [3] National Academy of Science, 000, Clearing the Air: Asthma and Indoor Exposures, Committee on the Assessment of Asthma and Indoor Air, Washington, D.C.: National Academy Press [4] NRC, 00, Re-evaluation of the indoor resuspension factor for the screening analysis of the building occupancy

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